The present invention relates to the recovery of magnetically recorded digital information and more particularly, the invention relates to improvements in the readout of digital information recorded on magnetic disks or diskettes.
Digital information is conventionally written or recorded on magnetic surfaces by placing on the surface a series of magnetic flux reversals. The spacing of these reversals alongside the track and/or the direction of the reversal is used as an encoding pattern. Various recording formats are known here (Manchester, Miller, triperiod, etc.). These codes and their development and evolution have in common the attempt to increase the data density (bits-per-inch rating). All codes and methods are limited by the (obvious) requirement that the reversals and their relative spacing must be recoverable. A vexing problem of long standing is the so-called peak displacement, which means that the read-back voltage produced by a transducer should have a peak when the transducer gap passes the center of the flux reversal area on the track. Consider three transitions (and read-back peaks) in sequence. If they are unequally spaced, the middle one will appear shifted towards the farther one, thus tending to "equalize" the spacing. For too closely spaced transitions, differences in spacing may become completely obliterated in that fashion. A companion problem, but so to speak on the opposite end, is posed by transitions which are spaced too far on account of too low a package density. Whenever transitions are spaced too far, flat portions occur in the read-back signal, in between the widely spaced peaks.
The entire magnetic record and read-back process can be analyzed as follows. The magnetic storage system can be represented in a model as a "black box," i.e., a quadrupole whose input is the recording voltage, whose output is the transducer read-back voltage, and whose content is the entire magnetic storage system. It was found that such a system acts like a low pass filter, and at least some of the read-back and recovery problems can be traced in that fasion. It has been proposed to use a so-called slimming filter in the recovery and play-back circuit of digital magnetic tape storage. The slimming filter has also been used in the output circuit and a flying head cooperating with a hard recording surface. The read-back voltage is passed through the slimming filter which boosts the frequencies in the range of the high frequency roll-off of the above-defined low pass filter. Of critical importance here is the requirement that the filter must not introduce a frequency-dependent phase shift among the various signal components because such a phase shift produces its own distortion.
Pursuant to further development in this field, and here particularly to the problem of data recovery from recordings on a flexible disk, it was discovered that a slimming filter of the type referred to above actually interfers with the recovery of digital data whenever the transitions or flux reversals on the magnetic recording track are physically spaced fairly wide. This occurs particularly in the outer tracks of a magnetic recording disk.